The use of analog modems to transport user data over switched-access telephone channels is common. As is known, these devices transport user data by transforming the user data into analog signals and then transmitting the analog signals to a remote modem by means of a switched-access telephone channel.
Recently digital modems have been developed. As is known, digital modems--similar to analog modems--may be used to transport user data over switched-access telephone channels. In contrast to the analog modems, however, these devices transport user data by sending digital signals to a remote modem by means of a telephone channel. Because these digital modems transmit and receive digital signals, they are incompatible with analog modems, which transmit and receive only analog signals.
The problem of establishing a switched-access data communications session between a first subscriber using an analog modem and a second subscriber using a digital modem is known. One common solution includes equipping the serving central office with an analog modem coupled to a digital modem, with the modem-terminal interface of the analog modem being cross-wired to the modem-terminal interface of the digital modem. Thus, the analog modem's receive data ("RD") and, transmit data ("TD") leads is respectively coupled to the digital modem's TD and RD leads. Typically such arrangements are called "modem pools."
With this modem-pooling arrangement, the first subscriber's analog modem is connected to the central-office analog modem, while the second subscriber's digital modem is connected to the central-office digital modem. The first subscriber's analog modem now transmits analog-type data signals to the central-office analog modem, which presents the corresponding user data on its RD lead. In turn, this user data is applied to the central-office digital modem by means of the digital modem's TD lead. In turn, the central-office digital modem converts the user data to digital signals which are then transmitted to the second subscriber's digital modem.
There is a problem with such modem pools, however. This problem occurs when the first subscriber's analog modem and the second subscriber's digital modem initially operate at different data rates. In this situation, it is desirable to adjust the data rate of the second subscriber's digital modem equal to the data rate of the first subscriber terminal and analog modem. As a result of adjusting the data rates of the second subscriber's terminal and digital modem, a data communication link between the first subscriber's terminal and the second subscriber's terminal is established, and a data communications session may now proceed.